It has long been conventional to measure the relative air speed between a moving object, such as an aircraft, and the free airstream through which the aircraft is flying by means of a mechanical instrument which in effect compares the kinetic pressure exerted by the moving airstream onto a first defined area facing said airstream with respect to the static pressure exerted on a second defined area generally perpendicular to said first defined area. Typically, such a prior art system employs Pitot tubes, pneumatic tubing and pressure transducers which are exposed to the external environment and subject to not only degraded performance caused by calibration changes, but also catastrophic failures as a result of accidental breakage. Furthermore, such a prior art type of airspeed measurement device physically protrudes into the airflow, with a resultant drag penalty.
An example of another type of basically mechanical airspeed measurement device may be mentioned, namely, the cup anemometer wherein a plurality of cups are mounted at the periphery of a rotating carrier such that successive cups catch the wind as the carrier rotates. The carrier is connected by means of a transmission to an electric generator or other suitable tachometer device. The faster the wind's velocity, the greater the force exerted upon the individual cups and the faster the carrier rotates.
The atmosphere contains many naturally occurring aerosols having a diameter on the order of from 0.1 to 10 micrometers. Examples of such aerosols are pollen and dust that naturally occur in the atmosphere. These aerosols tend to follow the motion of the atmosphere in which they are entrained and hence by observing the motion of such particles, it is possible to measure the velocity of the surrounding airstream.
Accordingly, optical techniques have also been utilized to measure wind. One such technique employed scattered sunlight as a source and cross-correlated the outputs of two radiometers having intersecting fields of view. Against localized flumes, such as the flume from a smokestack, it produced satisfactory results. A more sophisticated type of optical device measured the Doppler shift in light scattered by particles within the moving fluid to measure velocity along the system's optical axis.
There has also been used a so-called fringe laser velocimeter which measured a velocity component transverse to the instruments's line of sight by detecting the movement of aerosol particles through a layered pattern of interference fringes created by two intersecting beams of coherent laser light. Such a device has proven particularly accurate at relatively close ranges.
Another optical technique relied upon variations in refractive index moving with the wind across the line of sight of the instrument requiring a so-called double-ended system having a separate source and receiver.
If it is required to measure relative wind speeds in three dimensions, it would theoretically be sufficient to provide three separate systems with mutually orthogonal axes aimed at a common region of space. In practice, such an arrangement is extremely difficult to maintain in alignement and expensive to implement.
It has also been proposed to utilize simultaneously two dominant colors from a single laser to form two mutually orthogonal sets of fringe planes in a common detection volume but, nevertheless, readily distinguishable from each other so that it would be possible to measure two orthogonal velocity components in the plane transverse to the velocimeter's optical axis. Alternatively, polarization or modulation by means of acoustic-optic modulators may be utilized to distinguish the signals corresponding to the two components. It has also been proposed to combine two different types of velocimeters in the same system, so that two orthogonal velocity components may be measured simultaneously, one parallel to the system's main axis (by the Doppler method), and one, at right angles thereto (using the fringe method); by also using the above-mentioned two component techniques it would thus also theoretically be possible to measure simultaneously three orthogonal velocity components (one by the Doppler technique, two with the fringe method).
As examples of prior art devices employing some of the measurement techniques briefly described above, mention may be made of U.S. Pat. No. 3,744,910 entitled "Apparatus for Optical Investigations and Measurements with the Aid of Several Coherent Light Beams"; U.S. Pat. No. 3,915,572 entitled "Combined Dual Scatter, Local Oscillator Laser Doppler Velocimeter"; U.S. Pat. No. 3,966,324 entitled "Laser Doppler Anemometer"; U.S. Pat. No. 3,984,685 entitled "Wind Measurement System"; U.S. Pat. No. 4,148,585 entitled "Three-Dimensional Laser Doppler Velocimeter"; U.S. Pat. No. 4,167,329 entitled "Focussed Doppler Radar"; U.S. Pat. No. 4,168,906 entitled "Differential Doppler Velocity Sensor"; and U.S. Pat. No. 4,179,218 entitled "Particle Size Analyzer."
Other types of velocimeters are described in an article entitled "Laser Doppler Velocimeter Detection Systems for Gas Velocity Measurements" by R. M. Huffaker, appearing in Applied Optics for May 1970; in a paper entitled "A Laser Velocimeter System" by C. A. Greated et al., presented at the Electro-Optics '71 International Conference held in Brighton, England in March 1971 and published by Industrial and Scientific Conference Management Inc. of Chicago, Ill.; in a publication entitled "Laser Instrumentation for Field Flow Diagnostics" by J. D. Trolinger, published by the North Atlantic Treaty Organization Advisory Group for Aerospace Research and Development as Agardograph No. 186, dated March 1974; in an article entitled "Three-Component Optical Doppler Anemometer" by V. M. Kulybin appearing at pages 1425 et seq. of the Soviet Physics Journal for October 1975; in an anonymous Disclosure No. 14125 entitled "Holographic Velocimeter" contained in Research Disclosure for January 1976 published by Industrial Opportunities, Ltd. of Homewell, England; in an article entitled "A Laser Doppler Velocimeter which Measures the Three Components of Velocity" by R. N. Dubnistchev et al. appearing at page 129 et seq. of Optics and Laser Technology for June 1976; in an article entitled "Laser Velocimeter for Wind Tunnel Measurements" by A. Boutier et al., appearing at pages 441 et seq. of IEEE Transactions on Aerospace and Electronics Systems for May 1978; in United Kingdom Patent Specification No. 1,524,830 entitled "Improvements Relating to Flow Measurement Apparatus" and dated Septemeber 1978; in an article entitled "New 3-D Laser Doppler Velocimeter Using Cross-Bispectral Analysis," by Takuso Sato et al. appearing at page 3890 et seq. of Applied Optics for 15 December 1978; and in an article entitled "Atmospheric Laser Doppler Velocimetry: An Overview" by J. W. Bilbro, appearing at page 4 et seq. of Optical Engineering for July/August 1980.
However, taken as a whole, the known prior art does not teach or suggest how the above-mentioned optical and other related techniques may be utilized to provide a compact radiation fringe velocimeter for measuring in three dimensions. To the contrary, the known prior art suggest that at least one velocity component--that parallel to the velocimeter's main axis--should be measured by the above-described Doppler method which requires that a coherency of the radiation be maintained even after it has been scattered back towards the velocimeter, where it still must be combined with unscattered light from a referenced source related in frequency and coherency to the radiation impinging upon the object, thus requiring a radiation source that is coherent and which is capable of maintaining such coherency over relatively long distances. However, the know devices which satisfy such a criterion employ a CO.sub.2 laser having a relatively long wavelength that is not optimal for the relatively small aerosol particles that are naturally found in the earth's atmosphere.
Furthermore, such known prior art does not teach or suggest the considerable improvement in signal-to-noise ratio that results from utilizing a single transmit/receive lens window to focus on a single detection volume containing three sets of fringe planes oriented with respect to one another such that no two of their three respective normal vectors are either coplanar nor orthogonal with respect to each other while at the same time each of the three normal vectors has a significant component in the direction of the system's main axis as well as a transverse component at right angles thereto. In particular, it is not seen how the average artisan could construct an optical velocimeter device that would measure the velocity of aerosol particles or other scattering centers in the free airstream at a location spaced apart from the surface of the aircraft and thus for the most part free from any associated turbulence effects, etc. and which, nevertheless, could be constructed in a relatively compact fashion utilizing, if desired, a single viewing window or lens.
Accordingly, it is a general object of the present invention to provide a compact radiation fringe velocimeter for measuring in three dimensions that is adaptable to use aboard an aircraft. It is the related object to provide such a device which creates three sets of radiation fringe plane patterns at varying observation points that is sufficiently remote from the components of the device that the velocimeter may be fully contained within the aircraft and yet the velocity of aerosol particles or other scattering centers in the free airstream are essentially undisturbed by compression and other effects resulting from the passage of the aircraft through the airstream.
It is another related object to provide a velocimeter utilizing optical techniques such as the creation of radiation fringes at the sample location to measure aircraft flight parameters such as true airspeed, angle of attack, and side slip angle efficiently and accurately.
The teachings and disclosures contained in the above-referenced U.S. patents and the information provided by the other above-referenced publications may contribute to a better understanding of the background of the present invention, as well as of its scope, function and possible manners of implementation and use; accordingly, they are hereby incorporated in their entirety by reference the same as if fully set out herein.